Factor XIa is an enzyme which is purified from beef blood. It is prepared from serum after the blood has clotted in glass containers. It converts bovine Factor IX from the plasma proenzyme form to an enzymatically active form. Our research involves the preparation of Factor XIa in highly purified form, followed by the tagging of the active center with a radioactive inhibitor such as diisopropylphosphofluoridate or phenylmethylsulfonylfluoride. The radioactively labelled enzyme will then be degraded and the amino acid sequence of the active site of the enzyme will be determined. BIBLIOGRAPHIC REFERENCES: G. D. Lominac and H. S. Kingdon, "A Simple and Inexpensive High Pressure Liquid Chromatographic System as an Adjunct to Gas Chromatography in Identification of Amino Acid Phenylthiohydantoins", Arch. Biochem. Biophys., 173, 320-325 (1976). D. Raulais, J. Hagaman, D. A. Ontjes, R. L. Lundblad and H.S. Kingdon, "The Complete Amino Acid Sequence of Rat Thyrocalcitonin," European J. Biochem., 64, 607-611 (1976).